The aim of these investigations is to employ immunological methods to characterize the surface structures of human leukemic cells with the goals of increasing our understanding of the pathophysiology of the malignant process and of establishing a basis for the rational design of specific therapies. Employing the somatic cell hybridization technique, we have prepared potent monoclonal antibodies to the previously described common acute lymphoblastic leukemia antigen (CALLA) and to other less well-studied leukemia-associated antigens. Using monoclonal antibodies to CALLA to facilitate identification and isolation of the antigen, we will continue our investigations of this surface glycoprotein as a model for the study of the biological significance and diagnostic and therapeutic utility of other tumor-associated antigens. During the past year, we have shown that CALLA-specific monoclonal antibodies identify a cell surface glycoprotein having a molecular weight of approximately 100,000 daltons on 2 to 6% of nonmyeloid nucleated cells of normal adult bone marrow, on normal fibroblasts in tissue culture, and on cells of several nonhematopoeitic human tumor cell lines. Using fluoresceinated or iodinated J5 antibody to modulate expression of CALLA on cells of leukemia cell lines, we showed that antibody-antigen complexes undergo a temperature-dependent redistribution on the cell surface during modulation to form microaggregates. Antibody, as well as antigen, is then internalized. Studies of 35S-methonine-labeled cells indicate that synthesis of CALLA continues despite modulation of its surface expression by specific antibody, implying that the presence of CALLA on the cell surface reflects a dynamic equilibrium between the processes of surface expression of newly synthesized glycoprotein and its spontaneous and antibody-mediated clearance. These and related studies also should allow us to assess the feasibility of employing antibodies to tumor or tissue-associated antigens as therapeutic agents for the elimination of selective cell populations in vivo (via passive serotherapy) or in vitro (using antibody-treated bone marrow for autologous transplantation).